Seiryuu is the god of the East, where the Country of Kutou and our opposite seishis are located. It is represented by the Eastern Blue Dragon who also represents war. Seiryuu's seven seishi are Nakago, Suboshi, Amiboshi, Soi, Ashitare, Tomo, and Miboshi. They protect Yui, who became Seiryuu no Miko. By becoming the Priestess of Seiryuu, Yui became Miaka's (her best friend) enemy as both countries were on war and they were both competing to call on (Summoning) their god for protection.
Yui became Seiryuu no Miko when she was found lost on the streets of the inner city of Kutou by Nakago. She had just been attacked by some bandits when Nakago found her and told her he had rescued her from the men. Nakago had noticed Yui's foreign clothes and realised she was the girl of "legend" from the other world they were waiting for. They also believed on the legend that said when a country was on the brick of destruction, a girl from another world would appear to become their Miko and acquire the divine power of their god to lead the country. Yui refused to become their miko/priestess at the beginning, but changed her mind after she overheard Miaka say she had come back for Tamahome. Yui had been waiting for Miaka to come for her and when she heard this, she felt betrayed and decided to fight against Miaka and become her enemy. She was also influenced by Nakago's lies.
Visit Seiryuu no Miko and the Seiryuu Seishi to your left to learn more about them.